The 21-aminosteroid inhibitors of lipid peroxidation: reactions with lipid peroxyl and phenoxy radicals.

The 21-aminosteroids U74006F and U74500A have been examined for their ability to scavenge the lipid peroxyl (LOO.) and phenoxy (PhO.) radicals. Lipid peroxidation was followed by measuring the formation of linoleic acid hydroperoxide (LOOH; 18:200H) from linoleic acid during incubations in methanol at 37 degrees C. Initiation of lipid peroxidation was by the radical generator 2,2'-azobis(2,4-dimethylvaleronitrile; AMVN), which under the conditions employed, initiated LOOH formation at a constant rate of 22 microM/h with a kinetic chain length of 21. Alpha-tocopherol (alpha TC) nearly completely blocked the chain reaction by scavenging LOO., reducing its formation to that essentially attributable to initiation alone. The average inhibition rate constant kinh for alpha TC at 37 degrees C was calculated as 4.9 x 10(5) M-1 sec-1. U74006F or U74500A also inhibited LOOH formation, reducing its rate to a constant fraction of control in a concentration dependent manner. U74500A was a more potent scavenger of LOO. than U74006F; however, both compounds were considerably less potent than alpha TC based upon their respective kinh's at 37 degrees C. Similarly, alpha TC, U74006F and U74500A scavenged PhO.. As seen with LOO. scavenging, alpha TC was orders of magnitude more reactive toward PhO. than either 21-aminosteroid as judged by their respective second order rate constants (k2). Both U74006F and U74500A were degraded during their reaction with LOO. or PhO. to as yet uncharacterized product(s). The data indicate that while the 21-aminosteroids can scavenge lipid radicals, their activity in this regard is less than expected based upon their ability to inhibit iron dependent lipid peroxidation.